In the quest for hydrocarbon reservoirs, companies employ many data-gathering techniques. The most detailed, albeit localized, data comes from well logging. During the well-drilling process, or shortly thereafter, drillers pass logging instruments through the well bore to collect information about the surrounding formations. The information is traditionally collected in “log” form, i.e., a table, chart or graph of measured data values as a function of instrument position. The most sought-after information relates to the location and accessibility of hydrocarbon gases and fluids. Resistivity, density, and porosity logs have proven to be particularly useful for this purpose. These logs are “open hole” logs, i.e., log measurements that are taken before the formation face is sealed with tubular steel casing. Such logs can be collected by several methods including wireline logging and logging while drilling (LWD).
In wireline logging, a sonde is lowered into the borehole after some or the entire well has been drilled. The sonde hangs at the end of a long cable (a “wireline”) that provides mechanical support and an electrical connection between the sonde and logging equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations.
When the position information for the logging instrument includes both depth and orientation, the log can take the form of a two-dimensional “image” of the borehole wall. Imaging enables analysts to study the fine-scale structure of the penetrated formations, including stratifications, fractures, dip angles, rock texture, vugs, and other features and anomalies. Most imaging tools are wireline logging tools, though at least some LWD imaging tools have been proposed. See, e.g., U.S. Pat. No. 6,600,321 (Evans); U.S. Pat. No. 7,098,664 (Bittar); U.S. Pat. No. 7,272,504 (Akimov); U.S. Pat. No. 6,678,616 (Winkler). There do not appear to be any logging while drilling tools that can provide a sonic density image log.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereof are not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.